Quite a few amp manufacturers have adopted the "no-extension-cab" feature into their combo amps lately.
Trying to go around that always means changing (or defeating) the internal speaker unless a series connection is made.

From their perspective it's a genious move, from the customers perspective not as much.
Most combo amps in the lower price range suffer from quite a few inadequacies, enough IMO not to be useful in anything else than as-is (practice) use.
That's a shame really, because even though I'm not particularily fond of combo amps, the Deltalite II modded SWR WM10 I sometimes use is an absolute killer for its size.
I don't have a compact extension cab for it, but with that it would take care of most of my small venue gigging needs with a 4 string.

If You like the tone that the Orange gives you, I'd mate a plate amp with a cab of your choosing and use that as an active extension cab if and when you need more volume.

Ya, it's best to let your smaller inexpensive combos like that be smaller inexpensive combos like that and enjoy them for what they are, as mods on them like your proposed idea are turning a silk purse into a sow's ear.